No one has chimed in on your question yet Dan so I will throw in my 2 cents. My first 2nd gen truck was a 2000 Tracker with the 2 liter and auto transmission. I was really surprised by how lively it was and the good use of the transmission gearing for acceleration. My previous four trucks had been 1st generation all with the 1.6 liter either 8 or 16 valve engines... so there ya go. I put about 50K on this first 2nd gen truck. I DID NOT modify this truck. I sold it in its stock configuration to buy my next 2 liter trucks with the 5 speed trannys.
The "loss of power" is a subjective topic simply because you don't always 'feel it' until you begin to drive up a hill, or begin to accelerate to pass another vehicle... or when you are trail driving and suddenly have to give your truck alot more throttle to accomplish the same task that didn't require as much throttle before you changed tire size. Additionally, no two persons would probably describe the loss of power the same way and the concept of what is 'acceptable power loss' is equally arguable.
The automatic equipped Trackers come with the 4.88 gear sets in them (early 1.6 engine equipped trucks set aside). They also come with 27" (+/-) tires standard as do all the Trackers regardless of engine size equipped. Virtually any 31" tire on the market will vary from 30.4" to a full 31" in diameter (why?... I have no idea... but probably for a similar market reason as why current women's jeans in size 6... ten years ago would have actually been size 10) so you will be effectively reducing the stock factory mechanical advantage by 11% (+/-). The auto trannys absorb a degree of this additional wear but the power output at the wheels cannot be the same. Obviously, this loss won't be as noticeable on flat terrain driving as it will on rolling terrain, and hills/mountains. Add trail driving (if you intend to drive it off-road) and it would be even more noticeable. In addition the extra tire mass factors in as well. Some more than others.
The manual 2 liter engine Tracker trucks came standard with 4.62 gears (37 tooth ring gear). Replacing the 4.62s with 4.88 gears (2 more teeth on the ring gear = 39) returns most of the lost mechanical advantage when running a 28.5-29" tire. 5.12 gears (41 tooth ring gears) will return most of the lost mechanical advantage when running 30+" tires setting aside the additional weight of some of the heavier tires.
So using this same thought process... I'd offer that to run true 31" tires on an auto tranny equipped truck AND still enjoy comparable power and accelleration of the stock size tire configuration WHILE also reducing the excessive wear on the power train that upgrading to at least 5.38 R&P gear sets would be a decent compromise. In doing this you would be moving from a 12 bolt rear ring gear down to a 10 bolt ring gear and you would have to change the front axle side gears to 26 spline... but I'm thinking this would keep your power train close to the factory mechanical advantage AND keep your speedo reading fairly accurate.
However, on the other hand if you live on flat plains, don't intend to ride trails and don't mind losing 'some power' just stick the 31s on and drive it. Note your speedometer reading will be off significantly (I'm guessing in the neighborhood of 10MPH+/-).
2 door trucks may well be slightly more forgiving with slightly less weight.
